Recently, as buildings have become large-sized, demand for multi-air conditioners, in which an outdoor unit is connected to a plurality of indoor units, has been increased. In general, the individual indoor units of such a multi-air conditioner have different required cooling capacities and each of the indoor units is independently operated, such that the total cooling capacity obtained by summing up the required cooling capacities of all the indoor units is varied. Accordingly, in order to meet the variation of the total required cooling capacity, the capacity of a compressor is adjusted according to the variation of the total required cooling capacity, and the opening ratio of an electric expansion valve situated upstream of an indoor heat exchanger or evaporator is controlled for each of the indoor units.
As a compressor having a capacity to be varied according to the variation of a required cooling capacity, a variable-speed compressor is known. Such a variable-speed compressor adjusts the capacity of the compressor according to the variation of the required cooling capacity by varying the frequency of a current applied to a motor through an inverter control method and thus controlling the rotational speed of the motor. The conventional variable-speed compressor requires a circuit for controlling the speed of the motor according to the required cooling capacity. The control circuit has a converting unit for converting an AC power voltage into a DC power voltage, and an inverting unit for inverting a DC power voltage into an AC power voltage.
However, the conventional variable-speed compressor is disadvantageous in that its efficiency is deteriorated due to a significant loss of energy in the control circuit.
A Pulse Width Modulated (PWM) compressor is disclosed as another type of variable-capacity compressor in U.S. Pat. No. 6,047,557 and Japanese Patent Laid-open Publication No. 8-334094. The PWM compressor is effectively used in a refrigeration system having a plurality of refrigeration compartments or freezing compartments, but it is not equally applied to an air-conditioning system for buildings, which has a different control environment from the refrigeration system.
FIG. 8a is a view showing the control operation and suction pressure of a conventional compressor when a total required cooling capacity has been decreased in the unloading state of the compressor, and FIG. 8b is a view showing the control operation and suction pressure of the conventional compressor when a total required cooling capacity has been decreased in the loading state of the compressor.
Referring to FIG. 8a, when a total required cooling capacity has been decreased in a unloading state (a state of not discharging refrigerant, wherein a PWM valve is turned on) of a corresponding cycle (Nth cycle) (Ta), the amount of the refrigerant sucked into the compressor from an indoor units is decreased. However, the loading time (A) of the compressor is kept the same in the corresponding cycle (Nth cycle), so the compressor discharges more refrigerant than an actually required amount of refrigerant. Referring to FIG. 8b, when a total required cooling capacity has been decreased in a loading state (a state of discharging refrigerant, wherein the PWM valve is turned off) of a corresponding cycle (Nth cycle), the loading time (A) of the compressor is kept the same in the corresponding cycle (Nth cycle). Therefore, the compressor discharges more refrigerant than an actually required amount of refrigerant. Accordingly, the suction pressure of the compressor is decreased excessively in the corresponding cycle (Nth cycle) (refer to “D” in FIG. 8b).
Hence, in the prior art, even though the actually required cooling capacity is decreased in the corresponding cycle, the capacity of the compressor is not adjusted in the corresponding cycle. After the corresponding cycle is over, the capacity of the compressor is varied to correspond to the varied required cooling capacity.
As described above, if the air conditioner employs the PWM compressor, a loading time when refrigerant is discharged and an unloading time when refrigerant is not discharged are cyclically repeated during the operation of the compressor, such that the flow of the refrigerant occurs periodically in a cycle. Hence, if the capacity of the compressor is not swiftly adjusted to meet the total required cooling capacity, the suction pressure of the compressor may be rapidly decreased or increased, thus incurring damage to the compressor and causing the stoppage of the compressor operation.
Moreover, in spite of a decreased total required cooling capacity, if the compressor discharges excessive refrigerant, the indoor heat exchangers are apt to be overcooled or even frozen. So, the indoor units are obliged to operate periodically for preventing the respective indoor heat exchangers from being overcooled.